This invention is related to the invention disclosed in copending U.S. patent application having Ser. No. 419,367 of MacLane et al., now abandoned entitled "Method and Apparatus for Interlaced Printing", assigned to the same assignee as the present invention.
The preferred apparatus for and method of practicing the present invention is associated with an ink jet printer wherein a print head scans a print medium, most typically a sheet of paper, by shuttling back and forth across the sheet of paper or by moving continuously along the sheet of paper which is held against a rotating drum. Images are formed by selectively depositing ink deposits or pixels which are located on lines and in columns. The present invention however is equally applicable to any apparatus having an element which reciprocates along a predetermined path relative to a frame or other member.
Inherent in any such printing system is the requirement that the true position of the print head relative to a print medium on which an image is being printed must be known. To deal with this, conventional print-head carriages use incremental encoders, both linear and rotary. These encoders do not give an absolute position output, but instead give relative indications. Thus, some means must be found to establish an absolute reference point for the encoder whenever a system using such an encoder is first turned on or the absolute position is in question.
This is commonly done by using an index marker or flag that signifies the "home" position for the encoder. In the traditional motion-sensing application, a counter is connected to the output of the encoder, and detection of this index marker or flag is used to clear the counter. In the case of a printer using a linear encoder to determine carriage position, the usual method is to have an index marker at the extreme left position of the carriage. This establishes the "0" position of the carriage, and any position to the right of this corresponds to a positive number in the counter.
Printers must also be able to scan over a variety of different media or image widths. It is also desirable to recheck the encoder and connected counters against the reference, or index, marker relatively frequently so that count errors due to electronic noise, errors, and mechanical vibration do not cause cumulative errors. Ideally, the cumulative count should be checked against the zero reference every pass of the carriage.
In the case of a printer whose carriage is programmed to scan over a narrow media near the middle of its travel, the scan will never reach the index position and thus checking of the cumulative position error against the index will never occur unless special "zero" motions of the carriage are programmed.